1. Field of the Invention
The present invention relates in general to an elastic mounting structure, and more particularly to a fluid-filled elastic mounting structure suitably used as a mount to mount an engine of a motor vehicle, in particular, which is excellent in its characteristics of absorbing and damping vibrations at high and low frequencies.
2. Discussion of the Related Art
In the art of elastic mounting structures such as engine mounts of body mounts used in an automotive vehicles, there have been widely used various elastic mounts, each of which incorporates a rubber block interposed between two metal members to be connected to respective members of the vehicle. If such a rubber block is made of a rubber material having a relatively low dynamic spring rate or constant so as to adequately absorb or insulate vibrations at relatively high frequencies and effectively reduce the resulting noises, the loss factor of the rubber block and consequently the damping constant or coefficient are too low for the elastic mount to demonstrate sufficient characteristics required to damp comparatively low frequency vibrations. Described differently, elastic mounting structures like the engine mounts are required, to provide relatively soft spring characteristics (low dynamic spring rate) for effective isolation or reduction of high frequency vibrations and noises (in the neighborhood of 100 Hz), and at the same time to exhibit high damping characteristics (with a high loss factor) for effective damping of large-amplitude vibrations at low frequencies (in the neighborhood of 15 Hz).
In the meantime, in light of the above dichotomous requirements, there has been proposed the use of hydraulically damped or fluid-filled elastic mounts of various kinds, which utilize an elastic property of a rubber material, and a flow resistance of a hydraulic fluid through a fluid path in the form of a nozzle or orifice. A typical example of such a fluid-filled elastic damper or mounting structure is disclosed in Japanese Patent Application laid open in 1978 and 1982 under Publication Nos. 53-5376 and 57-9340, respectively. The disclosed mounting structure comprises: an elastic rubber block having therein a cavity open at its one end; a partition member cooperating with the elastic rubber block to define an operating chamber on one side of the partition member, which operating chamber includes the above-indicated cavity and is filled with an incompressible fluid; a flexible member such as a flexible diaphragm disposed on the other side of the partition member and cooperating with the partition member to define therebetween an equilibrium chamber filled with the incompressible fluid; fluid-path means for defining an orifice which communicates with the operating chamber and the equilibrium chamber; and a movable member in the form of a plate which is supported by the partition member such that the movable member is movable over a predetermined distance by a vibrational force applied to the fluid in the operating chamber, thereby providing effective damping of vibrations applied to the mounting structure.
In a fluid-filled elastic mounting structure, it is generally recognized that the fluid in the operating chamber functions as if the fluid were a rigid body. For this reason, even if the elastic rubber block is formed of a rubber having a low dynamic spring rate, the mounting structure incorporating the rubber block is difficult to provide effective vibration damping. In the elastic mouting structure as disclosed in the above-identified documents, however, movements of the movable plate permit a change in the volume of the fluid within the operating chamber (i.e., absorption of the fluid pressure), thus contributing to obviating the inconvenience indicated above. Further, provisions are made for inhibiting the movable plate from moving upon application of low frequency vibrations to the mounting structure, in order to allow a free flow of the fluid through the orifice from the operating chamber into the equilibrium chamber. In this case, the flow resistance through the orifice, and other factors result in increasing the loss factor of the mounting structure as a whole, and thus giving the mounting structure improved vibration damping capability at low frequencies.
As indicated above, the fluid-filled elastic mounting structure incorporating a movable member as disclosed in the above-identified Japanese Patent Applications provides an excellent solution to the inconvenience experienced on the conventional fluid-filled elastic mouts. However, the solution suffers from another problem that the mounting structure requires a complicated structural arrangement of the partition member, since the partition member must incorporate means for defining the orifice communicating with the operating and equilibrium chambers, and means for supporting the movable member movably over a predetermined distance. As a result, the cost of manufacture of the partition member is increased, and the procedure to assemble the partition member in the mounting structure is cumbersome and time-consuming. Further, the size of the partition member tends to be large, resulting in the mounting structure being bulky and heavy.
Another factor which causes increased size of the mounting structure is associated with the requirement of a considerably sufficient thickness of the partition member which is molded of a synthetic resin or die-cast of aluminum. The thickness must be large enough to provide the orifice for fluid communication with the operating and equilibrium chambers, and to provide a sufficient strength to withstand the maximum force applied thereto. Consequently, the size of the structure for supporting the partition member tends to be large, resulting in a further increase in the overall size of the mounting structure.